The invention relates to an ultra-miniature electro-mechanical safety and arming (S&A) device for gun-launched munitions. The invention incorporates mechanical logic for reliability and safety. The invention is typically fabricated using micro-electromechanical systems (MEMS) based technology and processes, but can also be fabricated or assembled using offshoot technologies such as plating, molding, plastic injection, ceramic casting, etc. An important application of the invention is in munition fuze safety and arming for gun-launched munitions, wherein launch (setback) acceleration and spin-induced centrifugal acceleration are sequentially detected, thresholded, and utilized to effectuate mechanical arming of a firetrain, and further wherein spurious and unacceptable inertial inputs such as transportation and handling vibration and mechanical shocks are rejected and do not effectuate mechanical arming of a firetrain.
To assure safety in the transportation, handling, and deployment of gun-fired and other explosive munitions, munition-fuze safety standards such as MIL-STD-1316 require that two unique and independent aspects of the launch environment must be detected in the weapon fuze system before the weapon can be enabled to arm. Examples of the aspects of the launch environment that are sensed electronically or mechanically in existing systems are: setback acceleration, rifling-induced spin, gun- or launch-tube exit, airflow, and flight apex. Munition fuzes also typically perform targeting functions, which can include electromagnetic or electrostatic target detection, range estimation, target impact detection, grazing impact detection, or timed delay.
Many of the above sensing functions can be performed either electronically or mechanically, as several examples illustrate. First, the velocity change due to setback acceleration during tube launch can be quantified using an accelerometer and an integrating circuit, or by using a mechanical integrator (See, e.g., U.S. Pat. No. 5,705,767). Second, the occurrence of setback acceleration or spin acceleration can be detected with a simple mechanical inertial switch such as a reed switch, or with a calibrated accelerometer and a threshold detection circuit. Third, target impact or grazing impact may be detected using a crush switch, an accelerometer with a threshold circuit, or a mechanical inertial switch. The best method to use for any of these functions in a given munition application depends on characteristics of the weapon system such as limitations of size, onboard system power, desired configuration, or on factors such as affordable cost, material selection and compatibility, requirements for safety, or requirements for reliability.
In some fuzing applications for small- and medium-caliber fuzes, several of the above requirements become especially important. For example, a mechanical S&A for a 20-mm bursting projectile should be inexpensive (on the order of several dollars when manufactured in large quantities), should be extremely small to allow room for payload (lethality), should preferably require no pre-launch power since the battery typically does not activate until launch, and should have an initiation circuit that operates from a low-voltage battery. The present invention strives to meet all of these requirements. By contrast, an alternative technology is that of electronic safety and arming (ESA), which is often implemented in missiles. However, an ESA is currently not feasible for medium-and small-caliber artillery because of the relatively large cost and volume associated with components such as a slapper detonator and its high-voltage fireset, low-inductance fire circuit, a micro-controller or ASIC, a battery, and the need for one or more environment sensors as inputs to fuze logic.
In the munition fuzing industry the need for ‘smarter’ or more effective weapons often requires additional space within the weapon for signal and guidance electronics, power management, and sensors, while the need for greater lethality or payload makes simultaneous demands on volume. One solution is in the further miniaturization of existing fuze functions, particularly in the area of mechanical safety and arming. There is also a need to reduce the cost of existing weapon functions to make munition systems more affordable. This need is felt acutely in small- and medium-caliber weapons because of the large numbers needed to be produced to support a fielded system.
With current trends, the domestic precision small-parts manufacturing industry involved in producing current-day ‘watchworks’-based mechanical S&As is diminishing or moving overseas, so that an alternative and economical domestic source is needed for future fuze components production. The present invention has the advantage that its manufacture draws on fabrication principles and techniques from the installed domestic infrastructure of the microelectronics industry and the partially-installed and rapidly growing MEMS fabrication and high-volume replication infrastructure.
The old methods of design, prototyping, and production involve S&A designs that are not optimal for integration with small- and medium-caliber munition fuzes. The old methods are too bulky, too expensive to manufacture, do not achieve a sufficient amount of safety, are limited in reliability, or are difficult or unsuitable to integrate with current sophisticated fuze technology that incorporates advanced target proximity detection, sensor integration, guidance, and global positioning system data integration. The old methods are similarly not optimal for large-caliber fuze applications, and for many of the same reasons.
The designs and technology incorporated in the present invention are highly desirable to accommodate the aforementioned competing demands for volume in ordnance that must contain increasingly sophisticated fuzing and guidance circuits, as well as larger warheads and payloads. The state of the art as represented by prior-art patents is inadequate for applications requiring extreme miniaturization, low cost, readiness for electronic integration, and the other advantages stated.
In general, prior-art mechanical S&As that are fabricated using conventional (non-MEMS-based) manufacturing processes are too large; are too expensive; use too many parts, often of dissimilar materials, that must be assembled; involve a domestic precision small-parts manufacturing industry that is shrinking and moving overseas;                involve tight clearances and dimensional tolerances that are expensive to fabricate and assure using conventional machining operations; involve materials and parts that require lubrication to reduce working friction, however, such lubricant can, over time, lose its lubricity, foul other parts, or become viscous; do not take advantage of recent micro-scale fabrication technologies that use principles and processes well known and widely utilized in the micro-electronic fabrication industry, e.g., optical masking directly from CAD layouts, optical exposure, chemical developing and rinsing, material plating or deposition, etc., to create three-dimensional mechanical structures.        
Some prior devices are shown in U.S. Pat. No. 6,167,809 issued on Jan. 2, 2001 to Robinson et al. and entitled “Ultra-Miniature Monolithic, Mechanical Safety-and-Arming Device for Projected Munitions” U.S. Pat. No. 6,308,631 to Smith et al.; U.S. Pat. No. 5,824,910 to Last et al.; U.S. Pat. No. 6,173,650 to Garvick et al.; U.S. Pat. No. 5,693,906 issued on Dec. 2, 1997 to Van Sloun and entitled “Electro-Mechanical Safety and Arming Device”; and U.S. Pat. No. 5,275,107 issued on Jan. 4, 1994 to Weber et al. and entitled “Gun-Launched Non-Spinning Safety and Arming Mechanism.”